1. Technical Field of the Disclosure
The present invention is related in general to electromagnetic actuators, and in particular to a moving voice coil actuator or transducer which does not utilize costly permanent magnets.
2. Description of the Related Art
A typical speaker used today utilizes a transducer that transforms varying electrical signals to corresponding audible signals. A conventional loudspeaker typically has a stationary support frame or housing, a diaphragm which is the movable membrane, a permanent magnet mounted on the housing, a coil support connected longitudinally to the diaphragm and a voice coil wound transversally on the coil support within the magnetic field of the magnet. As is known by those skilled in the art, vibrations are induced in the voice coil and diaphragm when an alternating source signal is supplied to the voice coil by an amplifier or the like and the induced magnetic field interacts with the magnetic field of the permanent magnet to alternately attract and repel the voice coil.
Such speakers work well but the primary disadvantage with the use of such conventional speakers is that they are relatively large and heavy due to the presence of the permanent magnet. Conventional permanent magnet speakers thus lose their attractiveness in many applications such as automobiles, airplanes, or any other application in which maintaining a lightweight system is a concern. Conventional permanent magnet speakers also begin to lose their attractiveness in applications such as high-powered woofers in which the sheer size and weight of the permanent magnet required for the speaker increases the weight of the speaker beyond acceptable limits. Furthermore, typical permanent magnets suitable for voice coil actuators of speakers are fabricated from Neodymium Iron Boron (NdFeB). For a given magnetic field strength, a field coil constructed of copper wire will be of lower cost than the NdFeB magnet.
In the early days of speaker technology when permanent magnets were not stable and had the tendency to lose their magnetism, speakers were manufactured with a continuously powered stationary electromagnet. Such speakers include a frame, an electromagnet mounted on the frame, a movable membrane mounted on the frame, and a voice coil mounted on the membrane and movable therewith. The coil of electromagnet or field coil is continuously powered by an internal DC power source such as a battery to produce a constant-polarity magnetic field for interaction with the alternating field in voice coil. Speakers employing permanent electromagnets were not only heavy and cumbersome because they required their own internal power source, but were also very inefficient and were replaced by permanent magnet speakers as soon as permanent magnet technology was suitably developed. The electromagnets of such speakers, being powered by an independent power sources, also were necessarily not excited in proportion to the source signals exciting the voice coils.
Permanent magnets like Neodymium have more recently replaced the earlier used bulky and power consuming electromagnets. Due to this and other reasons, there has recently been a greater than one hundred fold increase in the price of rare earth Neodymium. Thus, the price of Rare earth NdFeB magnets has increased to a level that makes the use of NdFeB permanent magnets not viable for many applications. The costs have further spread to even the substantially lower performance ceramic permanent magnets, which have also increased in price, although not to the same extent as rare earth NdFeB permanent magnets. Because of the current costs associated with them, it must be asked when it remains appropriate to use NdFeB in bulk form such as the way Neodymium is currently used in NdFeB magnets. It is not inconceivable that the use of rare earth elements such as Neodymium will be restricted to those applications where they are used in 2-D film and nano or micro-sheet formats as opposed to 3-D bulk applications such as the weighty and wasteful permanent magnets employed in today's loudspeakers and other motors.
Voice coil actuators are electromagnetic devices that provide force proportional to current applied to a coil. A typical voice coil actuator comprises a coil assembly and a magnet assembly. The magnet assembly comprises inner and outer yokes of soft magnet material, which each conduct magnetic flux and which together define an air gap in which the coil assembly is suspended for movement within the air gap. The magneto motive force, Fm, which drives this magnetic flux, can either be created by a permanent magnet or a field coil with electric current carrying wire encircling a soft magnetic material core.
In a typical voice coil actuator, an electrical current conductive coil is suspended at a zero current bias position within a magnetic field formed in a gap. The flux path of the field within this gap may be optimally radial with respect to the axis of the coil so that when an externally applied current conducts through the coil, a Lorentz force will be developed which displaces the coil axially from its zero current bias position. As is known, the Lorentz force is linearly proportional to the coil current. Different configurations of voice coil actuators can provide different shapes of Force vs. Stroke curves. In the known prior art related to the voice coil actuator, the magnetic field in the gap is derived from a permanent magnet core.
The most common magnet system topologies for permanent magnet voice coil actuators are radially symmetric or axisymmetric. The system topologies typical of micro-speakers include center magnet topology, ring-magnet topology, and double-magnet topology. These topologies are generally fully scalable to all common sizes of loudspeakers. The pot core magnet structure (center magnet topology) has the inherent advantage of lowest stray-field losses compared to the other topologies. In addition to the radially symmetric voice coil actuators described above, there are two additional electromagnetic voice coil actuator topologies commonly employed in loudspeakers namely, planar voice coil actuators and radial magnet voice coil actuators.
Magnet-less speakers without permanent magnets and that employ two electromagnetic coils, one of which is mounted on a movable membrane and the other of which is mounted on a fixed frame, are known in the art and were in fact in wide use prior to the advent of reliable permanent magnets. Recent advancements in the art provide a lightweight speaker constructed without permanent magnets by providing two coils, one of which is mounted on a movable membrane and the other of which is mounted on a fixed frame. The coils are mounted in close proximity to one another and excited by a common source signal from a common amplifier or the like in such a fashion that the electromagnetic fields created by the coils upon excitation interact to cause the coils to alternately attract and repel one another. One of the coils is fed with an excitation signal directly from the source. The other coil receives the source signal only indirectly, preferably via a bridge rectifier. The coils may take the form of conventional wound wires or, in a particularly sophisticated yet inexpensive embodiment, may be formed on a printed circuit board in the form of flat spirals. The resulting speaker is very lightweight and thus is well suited for use in automobiles, airplanes, and other applications in which weight minimization is important. However, the source signal is not split using digital signal processing for the purpose of feeding the two coils. Hence, there is no provision for providing a feedback signal to linearize the actuation force so that it is a faithful representation of the incoming audio signal.
One of the existing field coil actuators includes a magnetic flux conductive material case, an electrical current conductive field coil and two electrical current conductive moving coils uniquely arranged. The case has a first surface and a continuous channel disposed in said first surface. The channel has a pair of opposing walls. The field coil is disposed within the channel between the walls so that a gap remains between the walls above the field coil and another gap remains between the walls below the field coil. When a current is induced in the field coil, magnetic flux is developed across the gaps. The flux is confined substantially normal to the walls of the channel. The electrical current conductive moving coils are each disposed moveably in one of the gaps such that an electrical current in the coil develops a Lorentz force on each of the coils in a direction substantially normal to the current in the moving coil and the magnetic flux to displace the moving coil in response to the current in the moving coil. However, an independent power source is require to produce the constant magnetic flux and thus a DC current must to be applied to the field coil in order for the system to be operational.
Yet another conventional voice coil actuator includes a magnetic flux conductive material core, a magnet and an electrical current conductive coil. The core has a first surface and a continuous channel disposed in said first surface. The channel has a pair of opposing walls. The magnet is disposed in intimate contact with a first one of said walls and spaced from a second one of said walls so that a gap remains between the magnet and the second one of the walls. The magnet has a first face of a first magnetic polarity facing the first one of the walls and a second face of a second, opposite magnetic polarity facing the gap. The magnet is further spaced from a bottom of the channel so that magnetic flux is substantially normal from the second face across said gap to the second one of the walls. The electrical current conductive coil is disposed moveably in the gap such that an electrical current in the coil develops a magnetic force on the coil in a direction substantially normal to the magnetic flux to displace said coil in response to said magnetic force. However, a major difficulty with conventional single-ended planar magnet loudspeaker designs, as in this case, is the presence of low-frequency range distortion.
Various other loudspeakers exist that include a magnetic circuit having a magnet, a lower plate, and an upper plate. In the magnetic circuit, a gap between magnetic poles is formed between the upper plate and a center pole that stands straight from a center position of the lower plate. A voice coil is located within the gap. A center cap is mounted in the vicinity of an upper end of the coil bobbin. The speaker further comprises a diaphragm, an edge to be connected with an outer periphery of the diaphragm, and a bent portion formed in the vicinity of a border of a connecting portion connecting the diaphragm and the edge, wherein it is possible to prevent strength deterioration or damage of the diaphragm even when sound signals having large amplitude is inputted, and to prevent deterioration of acoustic characteristics. The bent portion is provided with a reinforcing portion in order to reinforce the bent portion. However, such loudspeakers employ permanent magnets in their voice coil actuators.
Based on the foregoing there is a demonstrable need for a voice coil actuator device that eliminates the use of permanent magnets and uses a low cost iron electromagnet structure. Such a needed voice coil actuator would comprise an integrated amplifier and field coil driver. The voice coil actuator device when integrated with loudspeaker drivers would operate as electrically efficient as class-D amplifiers, when driving contemporary loudspeaker drivers. The actuator would provide an efficient magnet circuit with low magnet flux loss from stray fields using pot core magnet structure geometry for the voice coil actuator's electromagnet. The device would use Soft Magnetic Composites (SMC) material for the pot core magnet structure that has Ferromagnetic or Super paramagnetic behavior with a near linear response to the input audio or other actuating signal, low eddy current losses in the electromagnet structure and AC operation from DC to 20 KHz. The needed device would include electronics signal processing that provides a linear response of the actuation force in both amplitude and frequency to the incoming audio signal and that has a bandwidth from DC to 20 KHz. The needed electronics signal processing units would employ negative feedback to linearize the actuation force so that it would be a faithful representation of the incoming audio signal. In addition, the needed device would employ efficient electronics amplification of audio signals using pulse width modulated (PWM) Class-D amplifiers for driving both the voice coil and field coil. Further, the device would offer significant weight reduction and efficient recirculation of the magnetic energy generated by the field coil and stored in the voice coil air gap. The needed device would also be able to integrate the magnet-less voice coil actuator with the electronic integrated circuits to provide a loudspeaker drive motor which can receive low level analog or digital noise free audio signals and power. Further, the needed device would extend the magnet-less methodology to other voice coil actuator topologies with linear motion and permanent magnet motors with rotational motion. Finally, the voice coil actuator would provide a method of encrypting copyrighted and other high-resolution audio works of art. The present invention overcomes prior art shortcomings by accomplishing these critical objectives.